This invention relates to powdered nickel based superalloy compositions and to a method including thermomechanical treatments for making articles having improved stress-rupture strength and resistance to time-dependant fatigue crack propagation.
It is well known that nickel based superalloys are extensively employed in high performance environments. Some of these alloys, and particularly alloys used in the rotating parts of gas turbines for aircraft, must exhibit a desirable balance between tensile, creep, and fatigue properties at elevated temperatures of 650.degree. C. or more. It is the creep properties as measured by the stress-rupture strength, and fatigue properties as measured by the resistance to fatigue crack propagation that are of concern in the alloy composition and processing method disclosed herein.
The desirable combination of properties of such alloys at high temperatures are at least in part due to the presence of a precipitate which has been designated as a gamma prime precipitate. More detailed characteristics of the phase chemistry of gamma prime are given in "Phase Chemistries in Precipitation-Strengthening Superalloy" by E. L. Hall, Y. M. Kouh, and K. M. Chang, Proceedings of 41st. Annual Meeting of Electron Microscopy Society of America, August 1983, p. 248.
A problem which has been recognized with many nickel based superalloys is that they are subject to formation of cracks either in fabrication or in use, and that the cracks can initiate or propagate while under stress as during use of the alloys in such structures as gas turbines and jet engines. The propagation or enlargement of cracks can lead to part fracture or other failure.
Fatigue is a process of progressive localized permanent structural change occurring in a material subjected to fluctuating stresses and strains that can culminate in cracks or complete fracture. It is well known that fatigue can cause failure of a material at stresses well below the stress the material is capable of withstanding under static load applications. What has been poorly understood until studies were conducted was that the formation and the propagation of cracks in structures formed from superalloys is not a monolithic phenomena in which all cracks are formed and propagated by the same mechanism, at the same rate, and according to the same criteria. The complexity of crack generation and propagation, and the interdependence of such propagation with the manner in which stress is applied is a subject on which important information has been gathered.
The period during which stress is applied to a member to develop or propagate a crack, the intensity of the stress applied, the rate of application and of removal of stress to and from the member and the schedule of this application was not well understood in the industry until a study was conducted under contract to the National Aeronautics and Space Administration. This study is reported in a technical report identified as NASA CR-165123 issued from the National Aeronautics and Space Administration in August 1980, identified as "Evaluation of the Cyclic Behavior of Aircraft Turbine Disk Alloys" Part II, Final Report, by B. A. Cowles, J. R. Warren and F. K. Hauke, and prepared for the National Aeronautics and Space Administration, NASA Lewis Research Center, Contract NAS3-21379.
A principal unique finding of the NASA sponsored study was that the rate of fatigue crack propagation was not uniform for all stresses applied nor to all manners of applying stress. More importantly, it was found that fatigue crack propagation actually varied with the frequency of the application of stress to the member where the stress was applied in a manner to enlarge the crack. More surprising still, was the finding from the NASA sponsored study that the application of stress at lower frequencies rather than at the higher frequencies previously employed in studies, actually increased the rate of crack propagation. In other words, the NASA study revealed that there was a time dependence in fatigue crack propagation. Further, the time-dependence of fatigue crack propagation was found to depend not on frequency alone but on the time during which the member was held under stress for a so-called hold-time.
The most undesirable time-dependent crack-growth behavior has been found to occur when a hold time is superimposed on a sine wave variation in stress. In such a case, a test sample may be subjected to stress in a sine wave pattern, but when the sample is at maximum stress, the stress is held constant for a hold-time. When the hold-time is completed the sine wave application of stress is resumed. According to this hold-time pattern, the stress is held for a designated hold-time each time the stress reaches a maximum in following the normal sine curve. This hold-time pattern of application of stress is a separate criteria for studying crack growth. This type of hold-time pattern was used in the NASA study referred to above.
Crack growth, i.e., the crack propagation rate, in high-strength alloy bodies is known to depend upon the applied stress (.sigma.) as well as the crack length (a). These two factors are combined by fracture mechanics to form one single crack growth driving force; namely, stress intensity K, which is proportional to .sigma..sqroot.a. Under the fatigue condition, the stress intensity in a fatigue cycle represents the maximum variation of cyclic stress intensity (.DELTA.K), i.e., the difference between K.sub.max and K.sub.min. At moderate temperatures, crack growth is determined primarily by the cyclic stress intensity (.DELTA.K) until the static fracture toughness K.sub.IC is reached. Crack growth rate is expressed mathematically as da/dN .alpha. (.DELTA.K).sup.n. N represents the number of cycles and n is a constant which is between 2 and 4. The cyclic frequency and the shape of the waveform are the important parameters determining the crack growth rate. For a given cyclic stress intensity, a slower cyclic frequency can result in a faster crack growth rate. This undesirable time-dependent behavior of fatigue crack propagation can occur in most existing high strength superalloys.
It has been determined that at low temperatures the fatigue crack propagation rate depends essentially on the intensity at which stress is applied to components and parts of such structures in a cyclic fashion. As is partially explained above, the crack growth rate at elevated temperatures cannot be determined simply as a function of the applied cyclic stress intensity .DELTA.K. Rather, the fatigue frequency can also affect the propagation rate. The NASA study demonstrated that the slower the cyclic frequency, the faster the crack grows per unit cycle of applied stress. It has also been observed that faster crack propagation occurs when a hold time is applied during the fatigue cycle. Time-dependence is a term which is applied to such cracking behavior at elevated temperatures where the fatigue frequency and hold time are significant parameters. The time-dependence of fatigue crack propagation is thermally activated so that the time-dependence can be significantly magnified at 760.degree. C. as compared to 650.degree. C.
Progress has been made in reducing the time-dependency of fatigue crack propagation rates in nickel based superalloys. For example U.S. Pat. Nos. 4,685,977 and 4,820,353 disclose nickel based superalloy compositions that are formed by traditional cast and wrought methods, and are shown to produce essentially time-independent fatigue crack propagation rates at 650.degree. C. In addition, the '353 patent discloses a supersolvus annealing method, and the '977 patent discloses forging above the solvus temperature and annealing above the recrystallization temperature to produce the time-independent fatigue crack propagation rates at 650.degree. C. U.S. Pat. No. 4,816,084 discloses a method for supersolvus annealing and slow cooling superalloy compositions having a gamma prime strengthening precipitate and prepared by powder metallurgy techniques. Such powder formed superalloys annealed by the method of the '084 patent are shown to produce essentially time-independent fatigue crack propagation rates up to 650.degree. C. The '084 patent is incorporated by reference herein.
To achieve increased engine efficiency and greater performance, constant demands are made for improvements in the strength and temperature capability of the alloys used in aircraft engines. One measure of temperature capability is the stress-rupture strength. A stress-rupture test is performed by applying a static load to a test specimen at an elevated temperature and measuring the time for the sample to fail or rupture. Alloys disclosed in the '977 patent discussed above were compared to Rene 95 by stress rupture testing at 760.degree. C. with a 75 ksi initial load. The alloys of the '977 patent had a rupture life of more than 300 hours as compared to less than 30 hours for Rene 95 samples prepared by powder metallurgy techniques. As used herein, the term ksi stands for kips per square inch or the unit of stress representing 1,000 pounds per square inch.
By stress-rupture testing at various loads and temperatures a given length of time can be determined for which the material will rupture over a range of temperatures and stresses. For example, a graph presenting the 100-hour stress-rupture strength of a material gives the temperature's and corresponding stress-rupture strength's at which the material ruptures after 100 hours in a stress-rupture test. A comparison of temperature capability between samples having different compositions or processing treatments can then be made by comparing the temperature at which the samples have the same 100-hour stress-rupture strength.
This invention specifically relates to superalloy compositions produced by powder metallurgy techniques and focuses on the stress-rupture strength and the time-dependence of fatigue crack propagation. Powder metallurgy refers to the fabrication of essentially fully dense stock or parts from metal powders. Fine metal powders are produced so that either each powder particle or a mixture of powders conforms to a final alloy composition. Loose powder aggregates are mechanically consolidated to form relatively dense compacts that are sintered at a temperature that causes strengthening and growth of interparticle bonds. The intrinsic strength of superalloy powders usually necessitates hot compaction in one or two steps combining the compaction and sintering operation.
It is an object of this invention to provide nickel based superalloy compositions and thermomechanical processes for forming the superalloys to produce essentially time-independent fatigue crack propagation rates and improved stress-rupture strength at elevated temperatures up to about 760.degree. C.
It is another object of this invention to provide nickel based superalloy compositions having increased temperature capability as shown by improved stress-rupture strength at elevated temperatures.
Another object of this invention is to provide superalloy compositions having a crack growth rate as small and as free of time-dependency as possible at temperatures up to about 760.degree. C.